Relative path support of portable VSCode instance

[i0air]

Checks

• I have checked existing feature requests.
• I have checked that this feature does not already exist.
• I am using the latest extension version.

This is a Feature

• This is a feature request and not a bug report.

Feature

background.windowBackgrounds can use relative path, like './data/user-data/background/img.png' or '{{pwd}}/background/img.png'

Reason

Portable VSCode use a folder "data" in exe path to save all settings. I want to use images in data/user-data/backgound folder, so I can take my VSCode everywhere without setting background again.